2. Pilbeam's Mechanical Ventilation
Physiological and Clinical
Applications
SEVENTH EDITION
J.M. Cairo, PhD, RRT, FAARC
Dean of the School of Allied Health Professions, Professor of
Cardiopulmonary Science, Physiology, and Anesthesiology, Louisiana State
University Health Sciences Center, New Orleans, Louisiana
3. Table of Contents
Cover image
Title page
Copyright
Dedication
Contributors
Preface
Acknowledgments
Chapter 1. Basic Terms and Concepts of Mechanical Ventilation
Physiological Terms and Concepts Related to Mechanical
Ventilation
Normal Mechanics of Spontaneous Ventilation
Lung Characteristics
4. Time Constants
Types of Ventilators and Terms Used in Mechanical Ventilation
Types of Mechanical Ventilation
Definition of Pressures in Positive Pressure Ventilation
Summary
Chapter 2. How Ventilators Work
Historical Perspective on Ventilator Classification
Internal Function
Power Source or Input Power
Control Systems and Circuits
Power Transmission and Conversion System
Summary
Chapter 3. How a Breath Is Delivered
Basic Model of Ventilation in the Lung During Inspiration
Factors Controlled and Measured During Inspiration
Overview of Inspiratory Waveform Control
Phases of a Breath and Phase Variables
Types of Breaths
Summary
5. Chapter 4. Establishing the Need for Mechanical Ventilation
Acute Respiratory Failure
Patient History and Diagnosis
Physiological Measurements in Acute Respiratory Failure
Overview of Criteria for Mechanical Ventilation
Possible Alternatives to Invasive Ventilation
Summary
Chapter 5. Selecting the Ventilator and the Mode
Noninvasive and Invasive Positive Pressure Ventilation: Selecting
The Patient Interface
Full and Partial Ventilatory Support
Breath Delivery and Modes of Ventilation
Modes of Ventilation
Bilevel Positive Airway Pressure
Additional Modes of Ventilation
Summary
Chapter 6. Initial Ventilator Settings
Determining Initial Ventilator Settings During Volume-Controlled
Ventilation
Initial Settings During Volume-Controlled Ventilation
6. Setting The Minute Ventilation: Special Considerations
Inspiratory Pause During Volume Ventilation
Determining Initial Ventilator Settings During Pressure Ventilation
Setting Baseline Pressure: Physiological Positive End-Expiratory
Pressure
Summary
Chapter 7. Final Considerations in Ventilator Setup
Selection of Additional Parameters and Final Ventilator Setup
Sensitivity Setting
Alarms
Periodic Hyperinflation or Sighing
Final Considerations In Ventilator Equipment Setup
Selecting the Appropriate Ventilator
Evaluation of Ventilator Performance
Chronic Obstructive Pulmonary Disease
Asthma
Neuromuscular Disorders
Closed Head Injury
Acute Respiratory Distress Syndrome
Acute Cardiogenic Pulmonary Edema and Congestive Heart
7. Failure
Summary
Chapter 8. Initial Patient Assessment
Documentation of The Patient–Ventilator System
The First 30 Minutes
Monitoring Airway Pressures
Vital Signs, Blood Pressure, and Physical Examination of The
Chest
Management of Endotracheal Tube and Tracheostomy Tube
Cuffs
Monitoring Compliance and Airway Resistance
Comment Section of The Ventilator Flow Sheet
Summary
Chapter 9. Ventilator Graphics
Relationship of Flow, Pressure, Volume, and Time
A Closer Look at Scalars, Curves, and Loops
Using Graphics to Monitor Pulmonary Mechanics
Assessing Patient–Ventilator Asynchrony
Advanced Applications
Summary
8. Chapter 10. Assessment of Respiratory Function
Noninvasive Measurements of Blood Gases
Capnography (Capnometry)
Exhaled Nitric Oxide Monitoring
Transcutaneous Monitoring
Indirect Calorimetry and Metabolic Measurements
Assessment of Respiratory System Mechanics
Measurements
Summary
Chapter 11. Hemodynamic Monitoring
Review of Cardiovascular Principles
Obtaining Hemodynamic Measurements
Interpretation of Hemodynamic Profiles
Clinical Applications
Summary
Chapter 12. Methods to Improve Ventilation in Patient–Ventilator
Management
Correcting Ventilation Abnormalities
Common Methods of Changing Ventilation Based on Paco2 and
Ph
9. Airway Clearance During Mechanical Ventilation
Secretion Clearance From an Artificial Airway
Administering Aerosols to Ventilated Patients
Types of Aerosol-Generating Devices
Postural Drainage and Chest Percussion
Flexible Fiberoptic Bronchoscopy
Additional Patient Management Techniques and Therapies in
Ventilated Patients
Fluid Balance
Psychological and Sleep Status
Patient Safety and Comfort
Transport of Mechanically Ventilated Patients Within an Acute
Care Facility
Summary
Chapter 13. Improving Oxygenation and Management of Acute
Respiratory Distress Syndrome
Basics of Oxygenation Using FIO2, PEEP Studies, and Pressure–
Volume Curves for Establishing Optimal PEEP
Introduction to Positive End-Expiratory Pressure and Continuous
Positive Airway Pressure
Peep Ranges
10. Indications for PEEP and CPAP
Initiating PEEP Therapy
Selecting The Appropriate PEEP/CPAP Level (Optimal PEEP)
Use of Pulmonary Vascular Pressure Monitoring with PEEP
Contraindications and Physiological Effects of PEEP
Weaning from PEEP
Acute Respiratory Distress Syndrome
Pathophysiology
Changes in Computed Tomogram with ARDS
ARDS as an Inflammatory Process
PEEP and the Vertical Gradient in ARDS
Lung-Protective Strategies: Setting Tidal Volume and Pressures
in ARDS
Long-Term Follow-Up on ARDS
Pressure–Volume Loops and Recruitment Maneuvers in Setting
PEEP in ARDS
Summary of Recruitment Maneuvers in ARDS
The Importance of Body Position During Positive Pressure
Ventilation
Additional Patient Cases
Summary
11. Chapter 14. Ventilator-Associated Pneumonia
Epidemiology
Pathogenesis of Ventilator-Associated Pneumonia
Diagnosis of Ventilator-Associated Pneumonia
Treatment of Ventilator-Associated Pneumonia
Strategies to Prevent Ventilator-Associated Pneumonia
Summary
Chapter 15. Sedatives, Analgesics, and Paralytics
Sedatives and Analgesics
Summary
Chapter 16. Extrapulmonary Effects of Mechanical Ventilation
Effects of Positive Pressure Ventilation on the Heart and Thoracic
Vessels
Adverse Cardiovascular Effects of Positive Pressure Ventilation
Factors Influencing Cardiovascular Effects of Positive Pressure
Ventilation
Beneficial Effects of Positive Pressure Ventilation on Heart
Function in Patients With Left Ventricular Dysfunction
Minimizing the Physiological Effects and Complications of
Mechanical Ventilation
12. Effects of Mechanical Ventilation on Intracranial Pressure, Renal
Function, Liver Function, and Gastrointestinal Function
Renal Effects of Mechanical Ventilation
Effects of Mechanical Ventilation on Liver and Gastrointestinal
Function
Nutritional Complications During Mechanical Ventilation
Summary
Chapter 17. Effects of Positive Pressure Ventilation on the Pulmonary
System
Lung Injury With Mechanical Ventilation
Effects of Mechanical Ventilation on Gas Distribution and
Pulmonary Blood Flow
Respiratory and Metabolic Acid–Base Status in Mechanical
Ventilation
Air Trapping (Auto-PEEP)
Hazards of Oxygen Therapy With Mechanical Ventilation
Increased Work of Breathing
Ventilator Mechanical and Operational Hazards
Complications of the Artificial Airway
Summary
Chapter 18. Troubleshooting and Problem Solving
13. Definition of the Term Problem
Protecting the Patient
Identifying the Patient in Sudden Distress
Patient-Related Problems
Ventilator-Related Problems
Common Alarm Situations
Use of Graphics to Identify Ventilator Problems
Unexpected Ventilator Responses
Summary
Chapter 19. Basic Concepts of Noninvasive Positive Pressure
Ventilation
Types of Noninvasive Ventilation Techniques
Goals of and Indications for Noninvasive Positive Pressure
Ventilation
Other Indications for Noninvasive Ventilation
Patient Selection Criteria
Equipment Selection for Noninvasive Ventilation
Setup and Preparation for Noninvasive Ventilation
Monitoring and Adjustment of Noninvasive Ventilation
Aerosol Delivery in Noninvasive Ventilation
14. Complications of Noninvasive Ventilation
Discontinuing Noninvasive Ventilation
Patient Care Team Concerns
Summary
Chapter 20. Weaning and Discontinuation From Mechanical
Ventilation
Weaning Techniques
Methods of Titrating Ventilator Support During Weaning
Closed-Loop Control Modes for Ventilator Discontinuation
Evidence-Based Weaning
Evaluation of Clinical Criteria for Weaning
Recommendation 1: Pathology of Ventilator Dependence
Recommendation 2: Assessment of Readiness for Weaning
Using Evaluation Criteria
Recommendation 3: Assessment During a Spontaneous
Breathing Trial
Recommendation 4: Removal of the Artificial Airway
Factors in Weaning Failure
Nonrespiratory Factors that may Complicate Weaning
Recommendation 6: Maintaining Ventilation in Patients with
Spontaneous Breathing Trial Failure
15. Final Recommendations
Recommendation 8: Weaning Protocols
Recommendation 9: Role of Tracheostomy in Weaning
Recommendation 10: Long-Term Care Facilities for Patients
Requiring Prolonged Ventilation
Recommendation 11: Clinician Familiarity with Long-Term Care
Facilities
Recommendation 12: Weaning in Long-Term Ventilation Units
Ethical Dilemma: Withholding and Withdrawing Ventilatory
Support
Summary
Chapter 21. Long-Term Ventilation
Goals of Long-Term Mechanical Ventilation
Sites for Ventilator-Dependent Patients
Patient Selection
Preparation for Discharge to The Home
Follow-Up and Evaluation
Equipment Selection for Home Ventilation
Complications of Long-Term Positive Pressure Ventilation
Alternatives to Invasive Mechanical Ventilation at Home
Expiratory Muscle AIDS and Secretion Clearance
16. Tracheostomy Tubes, Speaking Valves, and Tracheal Buttons
Ancillary Equipment and Equipment Cleaning for Home
Mechanical Ventilation
Summary
Chapter 22. Neonatal and Pediatric Mechanical Ventilation
Recognizing the Need for Mechanical Ventilatory Support
Goals of Newborn and Pediatric Ventilatory Support
Noninvasive Respiratory Support
Conventional Mechanical Ventilation
High-Frequency Ventilation
Weaning and Extubation
Adjunctive Forms of Respiratory Support
Summary
Chapter 23. Special Techniques Used in Ventilatory Support
Airway Pressure Release Ventilation
Other Names
Advantages of Airway Pressure Release Compared with
Conventional Ventilation
Disadvantages
Initial Settings21,32,33
17. Adjusting Ventilation and Oxygenation21,32,33
Discontinuation
High-Frequency Oscillatory Ventilation in the Adult
Technical Aspects
Initial Control Settings
Indication and Exclusion Criteria
Monitoring, Assessment, and Adjustment
Adjusting Settings to Maintain Arterial Blood Gas Goals
Returning to Conventional Ventilation
Heliox Therapy and Mechanical Ventilation
Gas Flow Through the Airways
Heliox in Avoiding Intubation and During Mechanical Ventilation
Postextubation Stridor
Devices for Delivering Heliox in Spontaneously Breathing Patients
Manufactured Heliox Delivery System
Heliox and Aerosol Delivery During Mechanical Ventilation
Monitoring the Electrical Activity of the Diaphragm and Neurally
Adjusted Ventilatory Assist
Review of Neural Control of Ventilation
Diaphragm Electrical Activity Monitoring
18. Neurally Adjusted Ventilatory Assist
Summary
Appendix A. Answer Key
Appendix B Review of Abnormal Physiological Processes
Appendix C Graphics Exercises
Index
Abbreviations
20. Knowledge and best practice in this field are constantly changing. As
new research and experience broaden our understanding, changes in
research methods, professional practices, or medical treatment may
become necessary.
Practitioners and researchers must always rely on their own
experience and knowledge in evaluating and using any information,
methods, compounds or experiments described herein. Because of
rapid advances in the medical sciences, in particular, independent
verification of diagnoses and drug dosages should be made. To the
fullest extent of the law, no responsibility is assumed by Elsevier,
authors, editors or contributors for any injury and/or damage to
persons or property as a matter of products liability, negligence or
otherwise, or from any use or operation of any methods, products,
instructions, or ideas contained in the material herein.
Library of Congress Control Number: 2019935218
Senior Content Strategist: Yvonne Alexopoulos
Senior Content Development Manager: Ellen Wurm-Cutter
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Printed in United States
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21.
22. Dedication
In Memoriam: Cora May Savoy
A generation goes, and a generation comes, but the earth
remains forever. The sun rises, and the sun goes down, and
hastens to the place where it rises. (Ecclesiastes 1:4–5)
23. Contributors
Robert M. DiBlasi, RRT-NPS, FAARC , Seattle Children’s
Hospital, Seattle, Washington
Terry L. Forrette, MHS, RRT, FAARC , Adjunct Associate
Professor of Cardiopulmonary Science, LSU Health Sciences Center,
New Orleans, Louisiana
Ancillary Contributor
Sandra T. Hinski, MS, RRT-NPS , Faculty, Respiratory Care
Division, Gateway Community College, Phoenix, Arizona
Reviewers
Stacia Biddle, MEd, RRT , Program Director of Respiratory
Therapy, Allied Health, The University of Akron, Akron, Ohio
Amy France, RRT , Assistant Professor and Respiratory Therapy
Program Leader, Shawnee State University, Portsmouth, Ohio
Mark Grzeskowiak, RRT, FAARC , Adjunct Instructor,
Respiratory Care Program, Orange Coast College, Costa Mesa,
California
John Rutkowski, MBA, MPA, RRT, FAARC, FACHE , Respiratory
Therapy Program director, County College of Morris, Randolph, New
Jersey
24. Preface
As with previous editions of Pilbeam’s Mechanical Ventilation, the goal
of this text is to provide clinicians with a strong physiological
foundation for making informed decisions when managing patients
receiving mechanical ventilation. The subject matter covered is
derived from current evidence-based practices and is written in a
manner that allows this text to serve as a resource for both students
and practicing clinicians. The seventh edition of Pilbeam’s Mechanical
Ventilation is presented in a concise manner that explains patient-
ventilator interactions. Beginning with the most fundamental concepts
and expanding to the more advanced topics, the text guides readers
through a series of essential concepts and ideas, building upon the
information as the reader progresses through the text.
It is apparent to critical care clinicians that implementing effective
interprofessional care plans is required to achieve successful
outcomes. Respiratory therapists are an integral part of effective
interprofessional critical care teams. Their expertise in the areas of
mechanical ventilation and respiratory care modalities is particularly
valuable considering the pace at which technological advances are
occurring in critical care medicine.
The application of mechanical ventilation principles to patient care
is one of the most sophisticated respiratory care applications used in
critical care medicine, making frequent reviewing helpful, if not
necessary. Pilbeam’s Mechanical Ventilation can be useful to all critical
care practitioners, including practicing respiratory therapists, critical
care residents and physicians, physician assistants, and critical care
nurse practitioners.
25. Organization
This edition, like previous editions, is organized into a logical
sequence of chapters and sections that build upon each other as a
reader moves through the book. The initial sections focus on core
knowledge and skills needed to apply and initiate mechanical
ventilation, whereas the middle and final sections cover specifics of
mechanical ventilation patient care techniques, including bedside
pulmonary diagnostic testing, hemodynamic testing, pharmacology of
patients receiving ventilation, and a concise discussion of ventilator-
associated pneumonia, as well as neonatal and pediatric mechanical
ventilatory techniques and long-term applications of mechanical
ventilation. The inclusion of some helpful appendixes further assists
the reader in the comprehension of complex material and an easy-
access Glossary defines key terms covered in the chapters.
26. Features
The valuable learning aids that accompany this text are designed to
make it an engaging tool for both educators and students. With clearly
defined resources in the beginning of each chapter, students can
prepare for the material covered in each chapter through the use of
Chapter Outlines, Key Terms, and Learning Objectives.
Along with the abundant use of images and information tables,
each chapter contains:
• Case Studies: Concise patient vignettes that list pertinent
assessment data and pose a critical thinking question to
readers to test their understanding of content learned.
Answers can be found in Appendix A.
• Critical Care Concepts: Short questions to engage the readers
in applying their knowledge of difficult concepts.
• Clinical Scenarios: More comprehensive patient scenarios
covering patient presentation, assessment data, and treatment
therapies. These scenarios are intended for classroom or group
discussion.
• Key Points: Highlight important information as key concepts
are discussed.
Each chapter concludes with:
• A bulleted Chapter Summary for ease of reviewing chapter
content
• Chapter Review Questions (with answers in Appendix A)
• A comprehensive list of References at the end of each chapter
for those students who wish to learn more about specific
topics covered in the text
Finally, several appendixes are included to provide additional
resources for readers. These include a Review of Abnormal
Physiological Processes, which covers mismatching of pulmonary
27. perfusion and ventilation, mechanical dead space, and hypoxia. A
special appendix on Graphic Exercises gives students extra practice in
understanding the interrelationship of flow, volume, and pressure in
mechanically ventilated patients. Answer Keys to Case Studies and
Critical Care Concepts featured throughout the text and the end-of-
chapter Review Questions can help the student track progress in
comprehension of the content.
This edition of Pilbeam’s Mechanical Ventilation has been updated to
reflect commonly used equipment and techniques to ensure it is in
step with the current modes of therapy. Case Studies, Clinical
Scenarios, and Critical Care Concepts are presented throughout the
text to emphasize this new information.
28. Learning Aids
Workbook
The Workbook for Pilbeam’s Mechanical Ventilation is an easy-to-use
guide designed to help the student focus on the most important
information presented in the text. The workbook features clinical
exercises directly tied to the learning objectives that appear in the
beginning of each chapter. Providing the reinforcement and practice
that students need, the workbook features exercises such as key term
crossword puzzles, critical thinking questions, case studies, waveform
analysis, and National Board for Respiratory Care (NBRC)-style
multiple-choice questions.
29. For Educators
Educators using the Evolve website for Pilbeam’s Mechanical Ventilation
have access to an array of resources designed to work in coordination
with the text and aid in teaching this topic. Educators may use the
Evolve resources to plan class time and lessons, supplement class
lectures, or create and develop student exams. These Evolve resources
offer:
• More than 800 NBRC-style multiple-choice test questions in
ExamView
• PowerPoint Presentation with more than 650 slides featuring
key information and helpful images
• An Image Collection of the figures appearing in the book
Jim Cairo, New Orleans, Louisiana
30. Acknowledgments
A number of individuals should be recognized for their contributions
to this project. I wish to offer my sincere gratitude to Sue Pilbeam for
her continued support. I also wish to thank Terry Forrette, MHS, RRT,
FAARC, for authoring the chapter on Ventilator Graphics; Rob
DiBlasi, RRT-NPS, FAARC, for authoring the chapter on Neonatal and
Pediatric Ventilation; and Theresa Gramlich, MS, RRT, and Paul
Barraza, RCP, RRT, for their contributions in earlier editions of this
text. I also wish to thank Sandra Hinski, MS, RRT-NPS, for authoring
the ancillaries that accompany this text and Amanda Dexter, MS, RRT,
and Gary Milne, BS, RRT, for their suggestions related to ventilator
graphics. As in previous editions, I want to express my sincere
appreciation to all of the Respiratory Therapy educators who
provided valuable suggestions and comments during the course of
writing and editing the seventh edition of Pilbeam’s Mechanical
Ventilation.
I would like to offer special thanks for the guidance provided by the
staff of Elsevier throughout this project, particularly Senior Content
Strategist, Yvonne Alexopoulos; Senior Content Development
Manager, Ellen Wurm-Cutter; Content Development Specialist,
Melissa Rawe; Project Manager, Janish Paul; and Publishing Services
Manager, Deepthi Unni. Their dedication to this project has been
immensely helpful and I feel fortunate to have had the opportunity to
work with such a professional group.
I particularly wish to thank my wife, Rhonda for always providing
love and support for me and all of our family. Her gift of
unconditional love and encouragement inspires me every day.
32. Basic Terms and Concepts of
Mechanical Ventilation
Physiological Terms and Concepts Related to Mechanical
Ventilation
Normal Mechanics of Spontaneous Ventilation
Ventilation and Respiration
Gas Flow and Pressure Gradients During
Ventilation
Units of Pressure
Definitions of Pressures and Gradients in the
Lungs
Transairway Pressure
Transthoracic Pressure
Transpulmonary Pressure
Transrespiratory Pressure
Lung Characteristics
Compliance
Resistance
Measuring Airway Resistance
Time Constants
Types of Ventilators and Terms Used in Mechanical Ventilation
Types of Mechanical Ventilation
Negative Pressure Ventilation
33. Positive Pressure Ventilation
High-Frequency Ventilation
Definition of Pressures in Positive Pressure Ventilation
Baseline Pressure
Peak Pressure
Plateau Pressure
Pressure at the End of Exhalation
Summary
LEARNING OBJECTIVES
On completion of this chapter, the reader will be able to do the
following:
1. Define ventilation, external respiration, and internal respiration.
2. Draw a graph showing how intrapleural and alveolar
(intrapulmonary) pressures change during spontaneous ventilation
and during a positive pressure breath.
3. Define the terms transpulmonary pressure, transrespiratory
pressure, transairway pressure, transthoracic pressure, elastance,
compliance, and resistance.
4. Provide the value for intraalveolar pressure throughout inspiration
and expiration during normal, quiet breathing.
5. Write the formulas for calculating compliance and resistance.
6. Explain how changes in lung compliance affect the peak pressure
measured during inspiration with a mechanical ventilator.
7. Describe the changes in airway conditions that can lead to
increased resistance.
8. Calculate the airway resistance given the peak inspiratory
pressure, a plateau pressure, and the flow rate.
34. 9. Using a figure showing abnormal compliance or airway
resistance, determine which lung unit will fill more quickly or with a
greater volume.
10. Compare several time constants, and explain how different time
constants will affect volume distribution during inspiration.
11. Give the percentage of passive filling (or emptying) for one, two,
three, and five time constants.
12. Briefly discuss the principle of operation of negative pressure,
positive pressure, and high-frequency mechanical ventilators.
13. Define peak inspiratory pressure, baseline pressure, positive
end-expiratory pressure, and plateau pressure.
14. Describe the measurement of plateau pressure.
KEY TERMS
• Acinus
• Airway opening pressure
• Airway pressure
• Alveolar distending pressure
• Alveolar pressure
• Ascites
• Auto-PEEP
• Bronchopleural fistulas
• Compliance
• Critical opening pressure
37. Physiological Terms and Concepts
Related to Mechanical Ventilation
The purpose of this chapter is to provide a brief review of the
physiology of breathing and a description of the pressure, volume,
and flow events that occur during the respiratory cycle. The effects of
changes in lung characteristics (e.g., respiratory compliance and
airway resistance) on the mechanics of breathing are also discussed.
38. Normal Mechanics of Spontaneous
Ventilation
Ventilation and Respiration
Spontaneous ventilation is simply the movement of air into and out of
the lungs. A spontaneous breath is accomplished by contraction of the
muscles of inspiration, which causes expansion of the thorax. During a
quiet inspiration, the diaphragm descends and enlarges the vertical
size of the thoracic cavity while the external intercostal muscles raise
the ribs slightly, increasing the circumference of the thorax.
Contraction of the diaphragm and external intercostal muscles
provides the energy to move air into the lungs and therefore perform
the “work” required to overcome the impedance offered by the lungs
and chest wall. During a maximal spontaneous inspiration, the
accessory muscles of breathing are also used to increase the volume of
the thorax.
During a normal quiet expiration, the inspiratory muscles simply
relax, the diaphragm moves upward, and the ribs return to their
resting position. The volume of the thoracic cavity decreases, and air
is forced out of the alveoli. To achieve a maximum expiration (below
the end-tidal expiratory level), the accessory muscles of expiration
must be used to compress the thorax. Box 1.1 lists the various
accessory muscles of breathing.
Respiration involves the exchange of oxygen and carbon dioxide
between an organism and its environment. Respiration is typically
divided into two components: external respiration and internal
respiration. External respiration involves the diffusion of oxygen and
carbon dioxide between the alveoli and the pulmonary capillaries.
Oxygenated blood leaving the pulmonary capillaries is carried by the
pulmonary veins to the left heart and distributed to the cells of the
body via the systemic arteries and capillaries. Internal respiration
occurs at the cellular level and involves the exchange of oxygen and
carbon dioxide between the systemic capillaries and the cells of the
39. body. At the cellular level, oxygen diffuses into the cells, where it is
used in the oxidation of available substrates (e.g., carbohydrates and
lipids) to produce energy. Carbon dioxide, which is a major by-
product of aerobic metabolism, diffuses out of the cells into the
systemic capillaries. Blood from the systemic capillaries is returned by
bulk flow via the systemic veins back to the right heart, the
pulmonary arteries, and the pulmonary capillaries.
BOX 1.1 Accessory Muscles of Breathing
Inspiration
Scalene (anterior, medial, and posterior)
Sternocleidomastoids
Pectoralis (major and minor)
Trapezius
Expiration
Rectus abdominis
External oblique
Internal oblique
Transverse abdominal
Serratus (anterior, posterior)
Latissimus dorsi
Gas Flow and Pressure Gradients During
Ventilation
For air to flow through a tube or airway, a pressure gradient must
exist (i.e., pressure at one end of the tube must be higher than pressure
at the other end of the tube). Air will always flow from the high-
pressure point to the low-pressure point.
40. Consider what happens during a normal quiet breath. Lung
volumes change as a result of gas flow into and out of the airways
caused by changes in the pressure gradient between the airway
opening and the alveoli. During a spontaneous inspiration,
contraction of the inspiratory muscles causes enlargement of the
thorax resulting in a decrease (more negative) in intrapleural and
alveolar pressure. The alveolar pressure therefore becomes less than
the pressure at the airway opening (i.e., the mouth and nose), and gas
flows into the lungs. Conversely, during a quiet expiration, relaxation
of the inspiratory muscles causes in a decrease in thoracic volume (i.e.,
diaphragm and external intercostal muscles return to their resting
position) and an increase in alveolar pressure. Gas flows out of the
lungs during expiration because the pressure in the alveoli is higher
than the pressure at the airway opening. It is important to recognize
that when the pressure at the airway opening and the pressure in the
alveoli are the same, as occurs at the end of expiration, bulk gas flow
does not occur because the pressures across the conductive airways
are equal (i.e., there is no pressure gradient).
Units of Pressure
Ventilating pressures are commonly measured in centimeters of water
pressure (cm H2O). These pressures are referenced to atmospheric
pressure, which is given a baseline value of zero. In other words,
although atmospheric pressure is 760 mm Hg or 1034 cm H2O (1 mm
Hg = 1.36 cm H2O) at sea level, atmospheric pressure is designated as
0 cm H2O. For example, when airway pressure increases by +20 cm
H2O during a positive pressure breath, the pressure actually increases
from 1034 to 1054 cm H2O. Other units of measure that are becoming
more widely used for gas pressures, such as arterial oxygen pressure
(PaO2) and arterial carbon dioxide pressure (PaCO2), are the torr (1
Torr = 1 mm Hg) and the kilopascal ([kPa]; 1 kPa = 7.5 mm Hg). The
kilopascal is used in the International System of units. (Box 1.2
provides a summary of common units of measurement for pressure.)
41. Definitions of Pressures and Gradients in the
Lungs
Airway opening pressure (Pawo) is most often called mouth pressure
(PM) or airway pressure (Paw) (Fig. 1.1). Other terms that are often
used to describe the airway opening pressure include upper-airway
pressure, mask pressure, and proximal airway pressure. 1 Unless
pressure is applied at the airway opening, Pawo is zero or atmospheric
pressure.
BOX 1.2 Pressure Equivalents
1 mm Hg = 1.36 cm H2O
1 kPa = 7.5 mm Hg
1 Torr = 1 mm Hg
1 atm = 760 mm Hg = 1034 cm H2O
A similar measurement is the pressure at the body surface (Pbs).
This is equal to zero (atmospheric pressure) unless the person is
placed in a pressurized chamber (e.g., hyperbaric chamber) or a
negative pressure ventilator (e.g., iron lung).
Intrapleural pressure (Ppl) is the pressure in the potential space
between the parietal and visceral pleurae. Ppl is normally about −5 cm
H2O at the end of expiration during spontaneous breathing. It is about
−10 cm H2O at the end of inspiration. Because Ppl is often difficult to
measure in a patient, a related measurement is used, the esophageal
pressure (Pes), which is obtained by placing a specially designed
balloon in the esophagus; changes in the balloon pressure are used to
estimate pressure and pressure changes in the pleural space. (See
Chapter 10 for more information about esophageal pressure
measurements.)
42.
43. FIG. 1.1 Various pressures and pressure gradients of the respiratory
system.
From Kacmarek RM, Stoller JK, Heuer AJ, eds. Egan’s Fundamentals
of Respiratory Care. 11th ed. St. Louis, MO: Elsevier; 2017.
Another commonly measured pressure is alveolar pressure (Palv).
This pressure is also called intrapulmonary pressure or lung pressure.
Alveolar pressure normally changes as the intrapleural pressure
changes. During spontaneous inspiration, Palv is about −1 cm H2O,
44. and during exhalation it is about +1 cm H2O.
Four basic pressure gradients are used to describe normal
ventilation: transairway pressure, transthoracic pressure,
transpulmonary pressure (or transalveolar pressure), and
transrespiratory pressure (Table 1.1; also see Fig. 1.1).
Transairway Pressure
Transairway pressure (PTA) is the pressure difference between the
airway opening and the alveolus: PTA = Pawo − Palv. It is therefore the
pressure gradient required to produce airflow in the conductive
airways. It represents the pressure that must be generated to
overcome resistance to gas flow in the airways (i.e., airway resistance).
Transthoracic Pressure
Transthoracic pressure (PW or PTT) is the pressure difference between
the alveolar space or lung and the body’s surface (Pbs): PW (or PTT) =
Palv − Pbs. It represents the pressure required to expand or contract the
lungs and the chest wall at the same time.
Transpulmonary Pressure
Transpulmonary pressure or transalveolar pressure (PL or PTP) is the
pressure difference between the alveolar space and the pleural space
(Ppl): PL (or PTP) = Palv − Ppl. PL is the pressure required to maintain
alveolar inflation and is therefore sometimes called the alveolar
distending pressure. 2-4 (NOTE: An airway pressure measurement
called the plateau pressure [Pplat] is sometimes substituted for Palv.
Pplat is measured during a breath-hold maneuver during mechanical
ventilation, and the value is read from the ventilator manometer. Pplat
is discussed in more detail later in this chapter.)
TABLE 1.1
Terms, Abbreviations, and Pressure Gradients for the Respiratory System
45. FIG. 1.2 The mechanics of spontaneous ventilation and the resulting
pressure waves (approximately normal values). During inspiration,
intrapleural pressure (Ppl) decreases to −10 cm H2O. During
exhalation, Ppl increases from −10 to −5 cm H2O. (See the text for
further description.)
All modes of ventilation increase PTP during inspiration, by either
decreasing Ppl (negative pressure ventilators) or increasing Palv by
increasing pressure at the upper airway (positive pressure
46. ventilators). During negative pressure ventilation, the pressure at the
body surface (Pbs) becomes negative and this pressure is transmitted
to the pleural space, resulting in a decrease (more negative) in
intrapleural pressure (Ppl) and an increase in transpulmonary
pressure (PL). During positive pressure ventilation, the Pbs remains
atmospheric, but the pressures at the airway opening (Pawo) and in the
conductive airways (airway pressure, or Paw) become positive.
Alveolar pressure (Palv) then becomes positive, and transpulmonary
pressure (PL) is increased. ∗
Transrespiratory Pressure
Transrespiratory pressure (PTR) is the pressure difference between the
airway opening and the body surface: PTR = Pawo − Pbs.
Transrespiratory pressure is used to describe the pressure required to
inflate the lungs during positive pressure ventilation. In this situation,
the body surface pressure (Pbs) is atmospheric and usually is given the
value zero; thus Pawo becomes the pressure reading on a ventilator
gauge (Paw).
Transrespiratory pressure has two components: transthoracic
pressure (the pressure required to overcome elastic recoil of the lungs
and chest wall) and transairway pressure (the pressure required to
overcome airway resistance). Transrespiratory pressure can therefore
be described by the equations PTR = PTT + PTA and (Pawo − Pbs) = (Palv −
Pbs) + (Paw − Palv).
Consider what happens during a normal, spontaneous inspiration
(Fig. 1.2). As the volume of the thoracic space increases, the pressure
in the pleural space (intrapleural pressure) becomes more negative in
relation to atmospheric pressures. (This is an expected result
according to Boyle’s law. For a constant temperature, as the volume
increases, the pressure decreases.) The intrapleural pressure drops
from about −5 cm H2O at end expiration to about −10 cm H2O at end
inspiration. The negative intrapleural pressure is transmitted to the
alveolar space, and the intrapulmonary, or alveolar (Palv), pressure
47. becomes more negative relative to atmospheric pressure. The
transpulmonary pressure (PL), or the pressure gradient across the
lung, widens (Table 1.2). As a result, the alveoli have a negative
pressure during spontaneous inspiration.
The pressure at the airway opening or body surface is still
atmospheric, creating a pressure gradient between the mouth (zero)
and the alveolus of about −3 to −5 cm H2O. The transairway pressure
gradient (PTA) is approximately (0 − [−5]), or 5 cm H2O. Air flows from
the mouth or nose into the lungs and the alveoli expand. When the
volume of gas builds up in the alveoli and the pressure returns to
zero, airflow stops. This marks the end of inspiration; no more gas
moves into the lungs because the pressure at the mouth and in the
alveoli equals zero (i.e., atmospheric pressure) (see Fig. 1.2).
During expiration, the muscles relax and the elastic recoil of the
lung tissue results in a decrease in lung volume. The thoracic volume
decreases to resting, and the intrapleural pressure returns to about −5
cm H2O. Notice that the pressure inside the alveolus during
exhalation increases and becomes slightly positive (+5 cm H2O). As a
result, pressure is now lower at the mouth than inside the alveoli and
the transairway pressure gradient causes air to move out of the lungs.
When the pressure in the alveoli and that in the mouth are equal,
exhalation ends.
TABLE 1.2
Changes in Transpulmonary Pressure a
Under Varying Conditions
48. a PL = Palv − Ppl.
b Applied pressure is +15 cm H2O.
49. Lung Characteristics
Normally, two types of forces oppose inflation of the lungs: elastic
forces and frictional forces. Elastic forces arise from the elastic
properties of the lungs and chest wall. Frictional forces are the result
of two factors: the resistance of the tissues and organs as they become
displaced during breathing and the resistance to gas flow through the
airways.
Two parameters are often used to describe the mechanical
properties of the respiratory system and the elastic and frictional
forces opposing lung inflation: compliance and resistance.
Compliance
The compliance (C) of any structure can be described as the relative
ease with which the structure distends. It can be defined as the inverse
of elastance (e), where elastance is the tendency of a structure to return
to its original form after being stretched or acted on by an outside
force. Thus C = 1/e or e = 1/C. The following examples illustrate this
principle. A balloon that is easy to inflate is said to be very compliant
(it demonstrates reduced elasticity), whereas a balloon that is difficult
to inflate is considered not very compliant (it has increased elasticity).
In a similar way, consider the comparison of a golf ball and a tennis
ball. The golf ball is more elastic than the tennis ball because it tends
to retain its original form; a considerable amount of force must be
applied to the golf ball to compress it. A tennis ball, on the other hand,
can be compressed more easily than the golf ball, so it can be
described as less elastic and more compliant.
In the clinical setting, compliance measurements are used to
describe the elastic forces that oppose lung inflation. More specifically,
the compliance of the respiratory system is determined by measuring
the change (Δ) of volume (V) that occurs when pressure (P) is applied
to the system: C = ΔV/ΔP. Volume typically is measured in liters or
milliliters and pressure in centimeters of water pressure. It is
50. important to understand that the compliance of the respiratory system
is the sum of the compliances of both the lung parenchyma and the
surrounding thoracic structures. In a spontaneously breathing
individual, the total respiratory system compliance is about 0.1 L/cm
H2O (100 mL/cm H2O); however, it can vary considerably, depending
on a person’s posture, position, and whether he or she is actively
inhaling or exhaling during the measurement. It can range from 0.05
to 0.17 L/cm H2O (50 to 170 mL/cm H2O). For intubated and
mechanically ventilated patients with normal lungs and a normal
chest wall, compliance varies from 40 to 50 mL/cm H2O in men and 35
to 45 mL/cm H2O in women to as high as 100 mL/cm H2O in either
gender (Key Point 1.1).
Key Point 1.1
Normal compliance in spontaneously breathing patients: 0.05 to 0.17
L/cm H2O or 50 to 170 mL/cm H2O
Normal compliance in intubated patients: Males: 40 to 50 mL/cm
H2O, up to 100 mL/cm H2O; Females: 35 to 45 mL/cm H2O, up to 100
mL/cm H2O
Changes in the condition of the lungs or chest wall (or both) affect
total respiratory system compliance and the pressure required to
inflate the lungs. Diseases that reduce the compliance of the lungs or
chest wall increase the pressure required to inflate the lungs. Acute
respiratory distress syndrome and kyphoscoliosis are examples of
pathological conditions associated with reductions in lung compliance
and thoracic compliance, respectively. Conversely, emphysema is an
example of a pulmonary condition in which pulmonary compliance is
increased as a result of a loss of lung elasticity. With emphysema, less
pressure is required to inflate the lungs.
Critical Care Concept 1.1 presents an exercise in which students can
test their understanding of the compliance equation.
51. Critical Care Concept 1.1 Calculate Pressure
Calculate the amount of pressure needed to attain a tidal volume of
0.5 L (500 mL) for a patient with a normal respiratory system
compliance of 0.1 L/cm H2O.
For patients receiving mechanical ventilation, compliance
measurements are made during static or no-flow conditions (e.g., this
is the airway pressure measured at end inspiration; it is designated as
the plateau pressure). Thus these compliance measurements are
referred to as static compliance or static effective compliance. The
tidal volume used in this calculation is determined by measuring the
patient’s exhaled volume near the patient connector (Fig. 1.3). Box 1.3
shows the formula for calculating static compliance (CS) for a
ventilated patient. Note that although this calculation technically
includes the recoil of the lungs and thorax, thoracic compliance
generally does not change significantly in a ventilated patient. (NOTE:
It is important to understand that if a patient actively inhales or
exhales during measurement of a plateau pressure, the resulting value
will be inaccurate. Active breathing can be a particularly difficult issue
when patients are tachypneic, such as when a patient is experiencing
respiratory distress.)
52. FIG. 1.3 A volume device (bellows) is used to illustrate the
measurement of exhaled volume. Ventilators typically use a flow
transducer to measure the exhaled tidal volume. The functional
residual capacity (FRC) is the amount of air that remains in the lungs
after a normal exhalation.
BOX 1.3 Equation for Calculating Static
Compliance
CS = (Exhaled tidal volume)/(Plateau pressure − EEP)
CS = VT/(Pplat − EEP) ∗
∗
EEP is the end-expiratory pressure, which some clinicians call the
baseline pressure; it is the baseline from which the patient
breathes. When positive end-expiratory pressure (PEEP) is
administered, it is the EEP value used in this calculation.
53. Resistance
Resistance is a measurement of the frictional forces that must be
overcome during breathing. These frictional forces are the result of the
anatomical structure of the airways and the tissue viscous resistance
offered by the lungs and adjacent tissues and organs.
As the lungs and thorax move during ventilation, the movement
and displacement of structures such as the lungs, abdominal organs,
rib cage, and diaphragm create resistance to breathing. Tissue viscous
resistance remains constant under most circumstances. For example,
an obese patient or one with fibrosis has increased tissue resistance,
but the tissue resistance usually does not change significantly when
these patients are mechanically ventilated. On the other hand, if a
patient develops ascites, or fluid accumulation in the peritoneal
cavity, tissue resistance increases.
The resistance to airflow through the conductive airways (airway
resistance) depends on the gas viscosity, the gas density, the length
and diameter of the tube, and the flow rate of the gas through the
tube, as defined by Poiseuille’s law. During mechanical ventilation,
viscosity, density, and tube or airway length remain fairly constant. In
contrast, the diameter of the airway lumen can change considerably
and affect the flow of the gas into and out of the lungs. The diameter
of the airway lumen and the flow of gas into the lungs can decrease as
a result of bronchospasm, increased secretions, mucosal edema, or
kinks in the endotracheal tube. The rate at which gas flows into the
lungs also can be controlled on most mechanical ventilators.
54. FIG. 1.4 Expansion of the airways during inspiration. (See the text for
further explanation.)
At the end of the expiratory cycle, before the ventilator cycles into
inspiration, normally no flow of gas occurs; the alveolar and mouth
pressures are equal. Because flow is absent, resistance to flow is also
absent. When the ventilator cycles on and creates a positive pressure
at the mouth, the gas attempts to move into the lower-pressure zones
in the alveoli. However, this movement is impeded or even blocked
by having to pass through the endotracheal tube and the upper
conductive airways. Some molecules are slowed as they collide with
the tube and the bronchial walls; in doing this, they exert energy
(pressure) against the passages, which causes the airways to expand
(Fig. 1.4); as a result, some of the gas molecules (pressure) remain in
the airway and do not reach the alveoli. In addition, as the gas
molecules flow through the airway and the layers of gas flow over
each other, resistance to flow, called viscous resistance, occurs.
The relationship of gas flow, pressure, and resistance in the airways
is described by the equation for airway resistance, Raw = PTA/flow,
55. where Raw is airway resistance and PTA is the pressure difference
between the mouth and the alveolus, or the transairway pressure (Key
Point 1.2). Flow is the gas flow measured during inspiration.
Resistance is usually expressed in centimeters of water per liter per
second (cm H2O/[L/s]). In normal, conscious individuals with a gas
flow of 0.5 L/s, resistance is about 0.6 to 2.4 cm H2O/(L/s) (Box 1.4).
The actual amount varies over the entire respiratory cycle. The
variation occurs because flow during spontaneous ventilation usually
is slower at the beginning and end of the cycle and faster in the
middle portion of the cycle. ∗
Key Point 1.2
Raw = (PIP − Pplat)/flow (where PIP is peak inspiratory pressure); or
Raw = PTA/flow; example:
BOX 1.4 Normal Resistance Values
Unintubated Patient
0.6 to 2.4 cm H2O/(L/s) at 0.5 L/s flow
Intubated Patient
56. Approximately 6 cm H2O/(L/s) or higher (airway resistance
increases as endotracheal tube size decreases)
Airway resistance is increased when an artificial airway is inserted.
The smaller internal diameter of the tube creates greater resistance to
flow (resistance can be increased to 5 to 7 cm H2O/[L/s]). As
mentioned, pathological conditions also can increase airway resistance
by decreasing the diameter of the airways. In conscious, unintubated
patients with emphysema and asthma, resistance may range from 13
to 18 cm H2O/(L/s). Still higher values can occur with other severe
types of obstructive disorders.
Several challenges are associated with increased airway resistance.
With greater resistance, a greater pressure drop occurs in the
conducting airways and less pressure is available to expand the
alveoli. As a consequence, a smaller volume of gas is available for gas
exchange. The greater resistance also requires that more force be
exerted to maintain adequate gas flow. To achieve this force,
spontaneously breathing patients use the accessory muscles of
inspiration. This generates more negative intrapleural pressures and a
greater pressure gradient between the upper airway and the pleural
space to achieve gas flow. The same occurs during mechanical
ventilation; more pressure must be generated by the ventilator to try
to “blow” the air into the patient’s lungs through obstructed airways
or through a small endotracheal tube.
Measuring Airway Resistance
Airway resistance pressure is not easily measured; however, the
transairway pressure can be calculated: PTA = PIP − Pplat. This allows
determination of how much pressure is delivered to the airways and
how much to alveoli. For example, if the peak pressure during a
mechanical breath is 25 cm H2O and the plateau pressure (i.e.,
pressure at end inspiration using a breath hold) is 20 cm H2O, the
pressure lost to the airways because of airway resistance is 25 cm H2O
− 20 cm H2O = 5 cm H2O. In fact, 5 cm H2O is about the normal
57. amount of pressure (PTA) lost to airway resistance (Raw) with a
proper-sized endotracheal tube in place. In another example, if the
peak pressure during a mechanical breath is 40 cm H2O and the
plateau pressure is 25 cm H2O, the pressure lost to airway resistance is
40 cm H2O − 25 cm H2O = 15 cm H2O. This value is high and indicates
an increase in Raw (see Box 1.4).
Many mechanical ventilators allow the therapist to choose a specific
constant flow setting. Monitors are incorporated into the user
interface to display peak airway pressures, plateau pressure, and the
actual gas flow during inspiration. With this additional information,
airway resistance can be calculated. For example, let us assume that
the flow is set at 60 L/min, the peak inspiratory pressure (PIP) is 40 cm
H2O, and the Pplat is 25 cm H2O. The PTA is therefore 15 cm H2O. To
calculate airway resistance, flow is converted from liters per minute to
liters per second (60 L/min = 60 L/60 s = 1 L/s). The values then are
substituted into the equation for airway resistance, Raw = (PIP −
Pplat)/flow:
For an intubated patient, this is an example of elevated airway
resistance. The elevated Raw may be caused by increased secretions,
mucosal edema, bronchospasm, or an endotracheal tube that is
too small.
Ventilators with microprocessors can provide real-time calculations
of airway resistance. It is important to recognize that where pressure
and flow are measured can affect the airway resistance values.
Measurements taken inside the ventilator may be less accurate than
those obtained at the airway opening. For example, if a ventilator
measures flow at the exhalation valve and pressure on the inspiratory
58. side of the ventilator, these values incorporate the resistance to flow
through the ventilator circuit and not just patient airway resistance.
Clinicians must therefore know how the ventilator obtains
measurements to fully understand the resistance calculation that is
reported.
Case Study 1.1 provides an exercise to test your understanding of
airway resistance and respiratory compliance measurements.
Case Study 1.1 Determine Static Compliance
(CS) and Airway Resistance (Raw)
An intubated, 36-year-old woman diagnosed with pneumonia is
being ventilated with a volume of 0.5 L (500 mL). The peak
inspiratory pressure is 24 cm H2O, Pplat is 19 cm H2O, and baseline
pressure is 0. The inspiratory gas flow is constant at 60 L/min (1 L/s).
What are the static compliance and airway resistance?
Are these normal values?
59. Time Constants
Regional differences in compliance and resistance exist throughout
the lungs. That is, the compliance and resistance values of a terminal
respiratory unit (acinus) may be considerably different from those of
another unit. Thus the characteristics of the lung are heterogeneous ,
not homogeneous. Indeed, some lung units may have normal
compliance and resistance characteristics, whereas others may
demonstrate pathophysiological changes, such as increased resistance,
decreased compliance, or both.
Alterations in C and Raw affect how rapidly lung units fill and
empty. Each small unit of the lung can be pictured as a small,
inflatable balloon attached to a short drinking straw. The volume the
balloon receives in relation to other small units depends on its
compliance and resistance, assuming that other factors are equal (e.g.,
intrapleural pressures and the location of the units relative to different
lung zones).
Fig. 1.5 provides a series of graphs illustrating the filling of the lung
during a quiet breath. A lung unit with normal compliance and
airway resistance will fill within a normal length of time and with a
normal volume (see Fig. 1.5, A). If the lung unit has normal resistance
but is stiff (low compliance), it will fill rapidly (see Fig. 1.5, B). For
example, when a new toy balloon is first inflated, considerable effort
is required to start the inflation (i.e., high pressure is required to
overcome the critical opening pressure of the balloon to allow it to
start filling). When the balloon inflates, it does so very rapidly at first.
It also deflates very quickly. Notice, however, that if a given pressure
is applied to a stiff lung unit and a normal unit for the same length of
time, a much smaller volume will be delivered to the stiff lung unit
(compliance equals volume divided by pressure) compared with the
volume delivered to the normal unit.
60. FIG. 1.5 (A) Filling of a normal lung unit. (B) A low-compliance unit,
which fills quickly but with less air. (C) Increased resistance; the unit
fills slowly. If inspiration were to end at the same time as in (A), the
volume in (C) would be lower.
Now consider a balloon (lung unit) that has normal compliance but
61. the straw (airway) is very narrow (high airway resistance) (see Fig.
1.5C). In this case the balloon (lung unit) fills very slowly. The gas
takes much longer to flow through the narrow passage and reach the
balloon (acinus). If gas flow is applied for the same length of time as
in a normal situation, the resulting volume is smaller.
The length of time lung units required to fill and empty can be
determined. The product of compliance (C) and resistance (Raw) is
called a time constant. For any value of C and Raw, the time constant
always equals the length of time (in seconds) required for the lungs to
inflate or deflate to a certain amount (percentage) of their volume. Box
1.5 shows the calculation of one time constant for a lung unit with a
compliance of 0.1 L/cm H2O and an airway resistance of 1 cm
H2O/(L/s). One time constant equals the amount of time it takes for
63% of the volume to be inhaled (or exhaled), two time constants
represent that amount of time for about 86% of the volume to be
inhaled (or exhaled), three time constants equal the time for about
95% to be inhaled (or exhaled), and four time constants is the time
required for 98% of the volume to be inhaled (or exhaled) (Fig. 1.6). 4-6
In the example in Box 1.5, with a time constant of 0.1 s, 98% of the
volume fills (or empties) the lungs in four time constants, or 0.4 s.
BOX 1.5 Calculation of Time Constant
Time constant = C × Raw
Time constant = 0.1 L/cm H2O × 1 cm H2O/(L/s)
Time constant = 0.1 s
In a patient with a time constant of 0.1 s, 63% of inhalation (or
exhalation) occurs in 0.1 s; that is, 63% of the volume is inhaled (or
exhaled) in 0.1 s, and 37% of the volume remains to be exchanged.
After five time constants, the lung is considered to contain 100% of
62. tidal volume to be inhaled or 100% of tidal volume has been exhaled.
In the example in Box 1.5, five time constants would equal 5 × 0.1 s, or
0.5 s. Thus, in half a second, a normal lung unit, as described here,
would be fully expanded or deflated to its end-expiratory volume
(Key Point 1.3).
Key Point 1.3
Time constants approximate the amount of time required to fill or
empty a lung unit.
Calculation of time constants is important when setting the
ventilator’s inspiratory time and expiratory time. An inspiratory time
less than three time constants may result in incomplete delivery of the
tidal volume. Prolonging the inspiratory time allows even distribution
of ventilation and adequate delivery of tidal volume. Five time
constants should be considered for the inspiratory time, particularly in
pressure ventilation, to ensure adequate volume delivery (see Chapter
2 for more information on pressure ventilation). It is important to
recognize, however, that if the inspiratory time is too long, the
respiratory rate may be too low to achieve effective minute
ventilation.
An expiratory time of less than three time constants may lead to
incomplete emptying of the lungs. This can increase the functional
residual capacity and cause trapping of air in the lungs. Some
clinicians think that using the 95% to 98% volume emptying level
(three or four time constants) is adequate for exhalation. 5,6 Exact time
settings require careful observation of the patient and measurement of
end-expiratory pressure to determine which time is better tolerated.
In summary, lung units can be described as fast or slow. Fast lung
units have short time constants and take less time to fill and empty.
Short time constants are associated with normal or low airway
resistance and decreased compliance, such as occurs in a patient with
interstitial fibrosis. It is important to recognize, however, that these
63. lung units will typically require increased pressure to achieve a
normal volume. In contrast, slow lung units have long time constants,
which require more time to fill and empty compared with a normal or
fast lung unit. Slow lung units have increased resistance or increased
compliance, or both, and are typically found in patients with
pulmonary emphysema.
It must be kept in mind that the lung is rarely uniform across
ventilating units. Some units fill and empty quickly, whereas others
do so more slowly. Clinically, compliance and airway resistance
measurements reflect a patient’s overall lung function, and clinicians
must recognize this fact when using these data to guide treatment
decisions.
64. FIG. 1.6 The time constant (compliance × resistance) is a measure of
how long the respiratory system takes to passively exhale (deflate) or
inhale (inflate).
From Kacmarek RM, Stoller JK, Heuer AJ, eds. Egan’s Fundamentals
of Respiratory Care, 11th ed, St. Louis, MO: Elsevier; 2017.
65. Types of Ventilators and Terms Used in
Mechanical Ventilation
Various types of mechanical ventilators are used clinically. The
following section provides a brief description of the terms commonly
applied to mechanical ventilation.
66. Types of Mechanical Ventilation
Three basic methods have been developed to mimic or replace the
normal mechanisms of breathing: negative pressure ventilation,
positive pressure ventilation, and high-frequency ventilation.
Negative Pressure Ventilation
Negative pressure ventilation (NPV) attempts to mimic the function of
the respiratory muscles to allow breathing through normal
physiological mechanisms. A good example of negative pressure
ventilators is the tank ventilator, or “iron lung.” With this device, the
patient’s head and neck are exposed to ambient pressure while the
thorax and the rest of the body are enclosed in an airtight container
that is subjected to negative pressure (i.e., pressure less than
atmospheric pressure). Negative pressure generated around the
thoracic area is transmitted across the chest wall, into the intrapleural
space, and finally into the intraalveolar space.
With negative pressure ventilators, as the intrapleural space
becomes negative, the space inside the alveoli becomes increasingly
negative in relation to the pressure at the airway opening
(atmospheric pressure). This pressure gradient results in the
movement of air into the lungs. In this way, negative pressure
ventilators resemble normal lung mechanics. Expiration occurs when
the negative pressure around the chest wall is removed. The normal
elastic recoil of the lungs and chest wall causes air to flow out of the
lungs passively (Fig. 1.7).
Negative pressure ventilators do provide several advantages. The
upper airway can be maintained without the use of an endotracheal
tube or tracheostomy. Patients receiving negative pressure ventilation
can talk and eat while being ventilated. Negative pressure ventilation
has fewer physiological disadvantages in patients with normal
cardiovascular function than does positive pressure ventilation. 7-10 In
hypovolemic patients, however, a normal cardiovascular response is
67. not always present. As a result, patients can have significant pooling
of blood in the abdomen and reduced venous return to the heart. 9,10
Additionally, difficulty gaining access to the patient can complicate
care activities (e.g., bathing and turning).
The use of negative pressure ventilators declined considerably in
the early 1980s, and currently they are rarely used in hospitals. Other
methods of creating negative pressure (e.g., chest cuirass, Poncho
wrap, and Porta-Lung) have been used in home care to treat patients
with chronic respiratory failure associated with neuromuscular
diseases (e.g., polio and amyotrophic lateral sclerosis). 8-13 More
recently, these devices have been replaced with noninvasive positive
pressure ventilators (NIV) that use a mask, a nasal device, or a
tracheostomy tube as a patient interface. Chapters 19 and 21 provide
additional information on the use of NIV and NPV.
68. FIG. 1.7 Negative pressure ventilation and the resulting lung
mechanics and pressure waves (approximate values). During
inspiration, intrapleural pressure drops from about −5 to −10 cm H2O
and alveolar (intrapulmonary) pressure declines from 0 to −5 cm H2O;
as a result, air flows into the lungs. The alveolar pressure returns to
zero as the lungs fill. Flow stops when pressure between the mouth
and the lungs is equal. During exhalation, intrapleural pressure
increases from about −10 to −5 cm H2O and alveolar (intrapulmonary)
pressure increases from 0 to about +5 cm H2O as the chest wall and
lung tissue recoil to their normal resting position; as a result, air flows
out of the lungs. The alveolar pressure returns to zero, and flow stops.
Positive Pressure Ventilation
Positive pressure ventilation (PPV) occurs when a mechanical
ventilator is used to deliver air into the patient’s lungs by way of an
endotracheal tube or positive pressure mask. For example, if the
pressure at the mouth or upper airway is +15 cm H2O and the
pressure in the alveolus is zero (end exhalation), the gradient between
69. the mouth and the lung is PTA = Pawo − Palv = 15 − (0), = 15 cm H2O.
Thus air will flow into the lung (see Table 1.1).
At any point during inspiration, the inflating pressure at the upper
(proximal) airway equals the sum of the pressures required to
overcome the resistance of the airways and the elastance of the lung
and chest wall. During inspiration, the pressure in the alveoli
progressively builds and becomes more positive. The resultant
positive alveolar pressure is transmitted across the visceral pleura,
and the intrapleural space may become positive at the end of
inspiration (Fig. 1.8).
At the end of inspiration, the ventilator stops delivering positive
pressure. Mouth pressure returns to ambient pressure (zero or
atmospheric). Alveolar pressure is still positive, which creates a
gradient between the alveolus and the mouth, and air flows out of the
lungs. See Table 1.2 for a comparison of the changes in airway
pressure gradients during passive spontaneous ventilation.
High-Frequency Ventilation
High-frequency ventilation uses above-normal ventilating rates with
below-normal ventilating volumes. There are three types of high-
frequency ventilation strategies: high-frequency positive pressure
ventilation (HFPPV), which uses respiratory rates of about 60 to 100
breaths/min; high-frequency jet ventilation (HFJV), which uses rates
between about 100 and 400 to 600 breaths/min; and high-frequency
oscillatory ventilation (HFOV), which uses rates into the thousands,
up to about 4000 breaths/min. In clinical practice, the various types of
high-frequency ventilation are better defined by the type of ventilator
used rather than the specific rates of each.
HFPPV can be accomplished with a conventional positive pressure
ventilator set at high rates and lower than normal tidal volumes. HFJV
involves delivering pressurized jets of gas into the lungs at very high
frequencies (i.e., 4 to 11 Hz or cycles per second). HFJV is
accomplished using a specially designed endotracheal tube adaptor
and a nozzle or an injector; the small-diameter tube creates a high-
70. velocity jet of air that is directed into the lungs. Exhalation is passive.
HFOV ventilators use either a small piston or a device similar to a
stereo speaker to deliver gas in a “to-and-fro” motion, pushing gas in
during inspiration and drawing gas out during exhalation. Ventilation
with high-frequency oscillation has been used primarily in infants
with respiratory distress and in adults or infants with open air leaks,
such as bronchopleural fistulas. Chapters 22 and 23 provide more
detail on the unique nature of this mode of ventilation.
71. Definition of Pressures in Positive
Pressure Ventilation
At any point in a breath cycle during mechanical ventilation, the
clinician can check the manometer, or pressure gauge, of a ventilator
to determine the airway pressure present at that moment. This
reading is measured either very close to the mouth (proximal airway
pressure) or on the inside of the ventilator, where it closely estimates
the airway opening pressure. ∗ A graph can be drawn that represents
each of the points in time during the breath cycle showing pressure as
it occurs over time. In the following section, each portion of the
graphed pressure or time curve is reviewed. These pressure points
provide information about the mode of ventilation and can be used to
calculate a variety of parameters to monitor patients receiving
mechanical ventilation.
72. FIG. 1.8 Mechanics and pressure waves associated with positive
pressure ventilation. During inspiration, as the upper airway pressure
rises to about +15 cm H2O (not shown), the alveolar (intrapulmonary)
pressure is zero; as a result, air flows into the lungs until the alveolar
pressure rises to about +9 to +12 cm H2O. The intrapleural pressure
rises from about 5 cm H2O before inspiration to about +5 cm H2O at
the end of inspiration. Flow stops when the ventilator cycles into
exhalation. During exhalation, the upper airway pressure drops to zero
as the ventilator stops delivering flow. The alveolar (intrapulmonary)
pressure drops from about +9 to +12 cm H2O to 0 as the chest wall and
lung tissue recoil to their normal resting position; as a result, air flows
out of the lungs. The intrapleural pressure returns to −5 cm H2O during
exhalation.
Baseline Pressure
Airway pressures are measured relative to a baseline value. In Fig. 1.9,
the baseline pressure is zero (or atmospheric), which indicates that no
additional pressure is applied at the airway opening during expiration
and before inspiration.
Sometimes the baseline pressure is higher than zero, such as when
the ventilator operator selects a higher pressure to be present at the
end of exhalation. This is called positive end-expiratory pressure
(PEEP) (Fig. 1.10). When PEEP is set, the ventilator prevents the
73. patient from exhaling to zero (atmospheric pressure). PEEP therefore
increases the volume of gas remaining in the lungs at the end of a
normal exhalation; that is, PEEP increases the functional residual
capacity. PEEP applied by the operator is referred to as extrinsic
PEEP. Auto-PEEP (or intrinsic PEEP), which is a potential side effect
of positive pressure ventilation, is air that is accidentally trapped in
the lung. Intrinsic PEEP usually occurs when a patient does not have
enough time to exhale completely before the ventilator delivers
another breath.
Peak Pressure
During positive pressure ventilation, the manometer rises
progressively to a peak pressure (PPeak). This is the highest pressure
recorded at the end of inspiration. PPeak is also called peak inspiratory
pressure (PIP) or peak airway pressure (see Fig. 1.9).
The pressures measured during inspiration are the sum of two
pressures: the pressure required to force the gas through the
resistance of the airways (PTA) and the pressure of the gas volume as it
fills the alveoli (Palv). ∗
Plateau Pressure
Another valuable pressure measurement is the plateau pressure. The
plateau pressure is measured after a breath has been delivered to the
patient and before exhalation begins. Exhalation is prevented by the
ventilator for a brief moment (0.5 to 1.5 s). To obtain this
measurement, the ventilator operator normally selects a control
marked “inflation hold” or “inspiratory pause.”
Plateau pressure measurement is similar to holding the breath at the
end of inspiration. At the point of breath holding, the pressures inside
the alveoli and mouth are equal (no gas flow). However, the
relaxation of the respiratory muscles and the elastic recoil of the lung
tissues are exerting force on the inflated lungs. This creates a positive
pressure, which can be read on the manometer as a positive pressure.
74. Because it occurs during a breath hold or pause, the manometer
reading remains stable and “plateaus” at a certain value (see Figs. 1.9
through 1.11 ). Note that the plateau pressure reading will be
inaccurate if the patient is actively breathing during the measurement.
75. FIG. 1.9 Graph of upper-airway pressures that occur during a positive
pressure breath. Pressure rises during inspiration to the peak
inspiratory pressure (PIP). With a breath hold, the plateau pressure can
be measured. Pressures fall back to baseline during expiration.
76. FIG. 1.10 Graph of airway pressures that occur during a mechanical
positive pressure breath and a spontaneous breath. Both show an
elevated baseline (positive end-expiratory pressure [PEEP] is +10 cm
H2O). To assist a breath, the ventilator drops the pressure below
baseline by 1 cm H2O. The assist effort is set at +9 cm H2O. PIP, Peak
inspiratory pressure; P TA , transairway pressure. (See text for further
explanation.)
Plateau pressure is often used interchangeably with alveolar
pressure (Palv) and intrapulmonary pressure. Although these terms
are related, they are not synonymous. The plateau pressure reflects
the effect of the elastic recoil on the gas volume inside the alveoli and
any pressure exerted by the volume in the ventilator circuit that is
acted upon by the recoil of the plastic circuit.
Pressure at the End of Exhalation
As previously mentioned, air can be trapped in the lungs during
mechanical ventilation if not enough time is allowed for exhalation.
The most effective way to prevent this complication is to monitor the
pressure in the ventilator circuit at the end of exhalation. If no
extrinsic PEEP is added and the baseline pressure is greater than zero
(i.e., atmospheric pressure), air trapping, or auto-PEEP, is present (this
78. FIG. 1.11 At baseline pressure (end of exhalation), the volume of air
remaining in the lungs is the functional residual capacity (FRC). At the
end of inspiration, before exhalation starts, the volume of air in the
lungs is the tidal volume (VT) plus the FRC. The pressure measured at
this point, with no flow of air, is the plateau pressure.
79. Summary
• Spontaneous ventilation is accomplished by contraction of the
muscles of inspiration, which causes expansion of the thorax,
or chest cavity. During mechanical ventilation, the mechanical
ventilator provides some or all of the energy required to
expand the thorax.
• For air to flow through a tube or airway, a pressure gradient
must exist (i.e., pressure at one end of the tube must be higher
than pressure at the other end of the tube). Air will always
flow from the high-pressure point to the low-pressure point.
• Several terms are used to describe airway opening pressure,
including mouth pressure, upper-airway pressure, mask pressure,
or proximal airway pressure. Unless pressure is applied at the
airway opening, Pawo is zero, or atmospheric pressure.
• Intrapleural pressure is the pressure in the potential space
between the parietal and visceral pleurae.
• The plateau pressure, which is sometimes substituted for
alveolar pressure, is measured during a breath-hold maneuver
during mechanical ventilation, and the value is read from the
ventilator manometer.
• Four basic pressure gradients are used to describe normal
ventilation: transairway pressure, transthoracic pressure,
transpulmonary pressure, and transrespiratory pressure.
• Two types of forces oppose inflation of the lungs: elastic forces
and frictional forces.
• Elastic forces arise from the elastance of the lungs and chest
wall.
• Frictional forces are the result of two factors: the resistance of
the tissues and organs as they become displaced during
breathing; and the resistance to gas flow through the airways.
• Compliance and resistance are often used to describe the
mechanical properties of the respiratory system. In the clinical
80. setting, compliance measurements are used to describe the
elastic forces that oppose lung inflation; airway resistance is a
measurement of the frictional forces that must be overcome
during breathing.
• The resistance to airflow through the conductive airways (flow
resistance) depends on the gas viscosity, the gas density, the
length and diameter of the tube, and the flow rate of the gas
through the tube.
• The product of compliance (C) and resistance (R) is called a
time constant. For any value of C and R, the time constant
approximates the time in seconds required to inflate or deflate
the lungs.
• Calculation of time constants is important when setting the
ventilator’s inspiratory time and expiratory time.
• Three basic methods have been developed to mimic or replace
the normal mechanisms of breathing: negative pressure
ventilation, positive pressure ventilation, and high-frequency
ventilation.
Review Questions (See Appendix A for answers.)
1. Using Fig. 1.12, draw a graph and show the changes in the
intrapleural and alveolar (intrapulmonary) pressures that
occur during spontaneous ventilation and during a
positive pressure breath. Compare the two.
2. Convert 5 mm Hg to cm H2O.
3. Which of the lung units in Fig. 1.13 receives more volume
during inspiration? Why? Which has a longer time
constant?
4. In Fig. 1.14, which lung unit fills more quickly? Which has
the shorter time constant? Which receives the greatest
volume?
81. 5. This exercise is intended to provide the reader with a
greater understanding of time constants. Calculate the
following six possible combinations. Then rank the lung
units from the slowest filling to the most rapid filling.
Because resistance is seldom better than normal, no
example is given that is lower than normal for this
particular parameter. (Normal values have been simplified
to make calculations easier.)
A. Normal lung unit: CS = 0.1 L/cm H2O; Raw = 1 cm
H2O/(L/s)
B. Lung unit with reduced compliance and normal airway
resistance: CS = 0.025 L/cm H2O; Raw = 1 cm H2O/(L/s)
C. Lung unit with normal compliance and increased
airway resistance: CS = 0.1 L/cm H2O; Raw = 10 cm
H2O/(L/s)
D. Lung unit with reduced compliance and increased
airway resistance: CS = 0.025 L/cm H2O; Raw = 10 cm
H2O/(L/s)
E. Lung unit with increased compliance and increased
airway resistance: CS = 0.15 L/cm H2O; Raw = 10 cm
H2O/(L/s)
F. Lung unit with increased compliance and normal airway
resistance: CS = 0.15 L/cm H2O; Raw = 1 cm H2O/(L/s)
82. FIG. 1.12 Graphing of alveolar and pleural pressures for spontaneous
ventilation and a positive pressure breath.
83. FIG. 1.13 Lung unit (A) is normal. Lung unit (B) shows an obstruction
in the airway.
6. 1 mm Hg =:
A. 1.63 cm H2O
B. 1.30 atm
C. 1.36 cm H2O
D. 1034 cm H2O
7. The pressure difference between the alveolus (Palv) and the
body surface (Pbs) is called:
A. Transpulmonary pressure
84. B. Transrespiratory pressure
C. Transairway pressure
D. Transthoracic pressure
8. Define elastance.
A. Ability of a structure to stretch
B. Ability of a structure to return to its natural shape after
stretching
85. FIG. 1.14 Lung unit (A) is normal. Lung unit (B) shows decreased
compliance (see text).
C. Ability of a structure to stretch and remain in that
position
D. None of the above
9. Which of the following formulas is used to calculate
compliance?
A. ΔV = C/ΔP
B. ΔP = ΔV/C
C. C = ΔV/ΔP
86. D. C = ΔP/ΔV
10. Another term for airway pressure is:
A. Mouth pressure
B. Airway opening pressure
C. Mask pressure
D. All of the above
11. Intraalveolar pressure (in relation to atmospheric
pressure) at the end of inspiration during a normal quiet
breath is approximately:
A. −5 cm H2O
B. 0 cm H2O
C. +5 cm H2O
D. 10 cm H2O
12. Which of the following is associated with an increase in
airway resistance?
A. Decreasing the flow rate of gas into the airway
B. Reducing the density of the gas being inhaled
C. Increasing the diameter of the endotracheal tube
D. Reducing the length of the endotracheal tube
13. Which of the following statements is true regarding
negative pressure ventilation?
A. Chest cuirass is often used in the treatment of
hypovolemic patients.
B. Tank respirators are particularly useful in the treatment
of burn patients.
C. The incidence of alveolar barotrauma is higher with
87. these devices compared with positive pressure
ventilation.
D. These ventilators mimic normal breathing mechanics.
14. PEEP is best defined as:
A. Zero baseline during exhalation on a positive pressure
ventilator
B. Positive pressure during inspiration that is set by the
person operating the ventilator
C. Negative pressure during exhalation on a positive
pressure ventilator
D. Positive pressure at the end of exhalation on a
mechanical ventilator
15. Which of the following statements is true regarding
plateau pressure?
A. Plateau pressure normally is zero at end inspiration.
B. Plateau pressure is used as a measure of alveolar
pressure.
C. Plateau pressure is measured at the end of exhalation.
D. Plateau pressure is a dynamic measurement.
16. One time constant should allow approximately what
percentage of a lung unit to fill?
A. 37%
B. 100%
C. 63%
D. 85%
17. A patient has a PIP of 30 cm H2O and a Pplat of 20 cm H2O.
Ventilator flow is set at a constant value of 30 L/min. What
88. is the transairway pressure?
A. 1 cm H2O
B. 0.33 cm H2O
C. 20 cm H2O
D. 10 cm H2O
89. References
1. Kacmarek R.M. Physiology of ventilatory
support. In: Kacmarek R.M, Stoller J.K, Heuer A.J, eds.
Egan’s fundamentals of respiratory care . ed 11. St.
Louis, MO: Elsevier; 2017:1016–1057.
2. Sanborn W.G. Monitoring respiratory mechanics
during mechanical ventilation: where do the signals
come from? Respir Care . 2005;50(1):28–54.
3. Hess D.R. Respiratory mechanics in mechanically
ventilated patients. Respir Care . 2014;59(11):1773–
1794.
4. Campbell E.J.M, Agostoni E, Davis J.N. The
respiratory muscles, mechanics and neural control . ed
2. London: Whitefriars Press; 1970.
5. Chatburn R.L, Volsko T.A. Mechanical
ventilators. In: Kacmarek R.M, Stoller J.K, Heuer A.J, eds.
Egan’s fundamentals of respiratory care . ed 11. St.
Louis, MO: Elsevier; 2017.
6.
Brunner J.X, Laubschre T.P, Banner M.J, Iotti G, Braschi A.
method to measure total expiratory time constant
based on passive expiratory flow-volume curve. Crit
Care Med . 1995;23(6):1117–1122.
7. Marks A, Asher J, Bocles L, et al. A new ventilator
assister for patients with respiratory acidosis. N Engl
J Med . 1963;268(2):61–68.
8. Hill N.S. Clinical applications of body ventilators.
Chest . 1986;90:897–905.
9. Kirby R.R, Banner M.J, Downs J.B. Clinical
90. applications of ventilatory support . ed 2. New York,
NY: Churchill Livingstone; 1990.
10. Corrado A, Gorini M. Negative pressure
ventilation. In: Tobin M.J, ed. Principles and practice of
mechanical ventilation . ed 3. New York, NY: McGraw-
Hill; 2013.
11. Holtackers T.R, Loosbrook L.M, Gracey D.R. The use
of the chest cuirass in respiratory failure of
neurologic origin. Respir Care . 1982;27(3):271–275.
12. Hansra I.K, Hill N.S. Noninvasive mechanical
ventilation. In: Albert R.K, Spiro S.G, Jett J.R, eds.
Clinical respiratory medicine . ed 3. Philadelphia,
PA: Mosby; 2008.
13. Splaingard M.L, Frates R.C, Jefferson L.S, et al. Home
negative pressure ventilation: report of 20 years of
experience in patients with neuromuscular disease.
Arch Phys Med Rehabil . 1983;66:239–242.
∗ The definition of transpulmonary pressure varies in research articles
and textbooks. Some authors define it as the difference between
airway opening pressure and pleural pressure, whereas others define
transpulmonary pressure as the pressure difference between airway
pressure and pleural pressure. This latter definition implies that
airway pressure is the pressure exerted by the lungs during a breath-
hold maneuver, that is, under static (no flow) conditions.4
∗ The transairway pressure (PTA) in this equation sometimes is
referred to as ?P, the difference between peak inspiratory pressure
(PIP) and Pplat. (See the section on defining pressures in positive
pressure ventilation.)
∗ During mechanical ventilation, proximal airway pressure is not
typically measured at the airway opening because of accumulation of
secretions and technical errors can alter sensor measurements.
91. Current-generation intensive care unit mechanical ventilators measure
airway pressure (Paw) using a sensor positioned proximal to the
expiratory valve, which is closed during the inspiration.2The
ventilator manometer pressure displayed on the user interface of the
ventilator is typically designated as airway pressure (Paw).
2
∗ At any point during inspiration, gauge pressure equals PTA + Palv.
The gauge pressure also will include pressure associated with PEEP.
93. How Ventilators Work
Historical Perspective on Ventilator Classification
Internal Function
Power Source or Input Power
Electrically Powered Ventilators
Pneumatically Powered Ventilators
Positive and Negative Pressure Ventilators
Control Systems and Circuits
Open-Loop and Closed-Loop Systems to Control
Ventilator Function
Control Panel (User Interface)
Pneumatic Circuit
Internal Pneumatic Circuit
External Pneumatic Circuit
Power Transmission and Conversion System
Compressors (Blowers)
Volume Displacement Designs
Volume Flow-Control Valves
Summary
94. LEARNING OBJECTIVES
On completion of this chapter, the reader will be able to do the
following:
1. List the basic types of power sources used for mechanical
ventilators.
2. Give examples of ventilators that use an electrical and a
pneumatic power source.
3. Explain the difference in function between positive and negative
pressure ventilators.
4. Distinguish between a closed-loop and an open-loop system.
5. Define user interface.
6. Describe a ventilator’s internal and external pneumatic circuits.
7. Discuss the difference between a single-circuit and a double-
circuit ventilator.
8. Identify the components of an external circuit (patient circuit).
9. Explain the function of an externally mounted exhalation valve.
10. Compare the functions of the three types of volume
displacement drive mechanisms.
11. Describe the function of the proportional solenoid valve.
KEY TERMS
• Closed-loop system
• Control system
• Double-circuit ventilator
• Drive mechanism
95. • External circuit
• Internal pneumatic circuit
• Mandatory minute ventilation
• Microprocessors
• Open-loop system
• Patient circuit
• Single-circuit ventilator
• User interface
Clinicians caring for critically ill patients receiving ventilatory support
must have a basic understanding of the principles of operation of
mechanical ventilators. This understanding should focus on patient-
ventilator interactions (i.e., how the ventilator interacts with the
patient’s breathing pattern, and how the patient’s lung condition can
affect the ventilator’s performance). Many different types of
ventilators are available for adult, pediatric, and neonatal care in
hospitals; for patient transport; and for home care. Mastering the
complexities of each of these devices may seem overwhelming at
times. Fortunately, ventilators have a number of properties in
common, which allow them to be described and grouped accordingly.
An excellent way to gain an overview of a particular ventilator is to
study how it functions. Part of the problem with this approach,
however, is that the terminology used by manufacturers and authors
varies considerably. The purpose of this chapter is to address these
terminology differences and provide an overview of ventilator
function as it relates to current standards. 1-3 It does not attempt to
review all available ventilators. For models not covered in this
discussion, the reader should consult other texts and the literature
provided by the manufacturer. 3 The description of the “hardware”
components of mechanical ventilators presented in this chapter
should provide clinicians with a better understanding of the
97. Historical Perspective on Ventilator
Classification
The earliest commercially available ventilators used in the clinical
setting (e.g., the Mörch and the Emerson Post-Op) were developed in
the 1950s and 1960s. These devices originally were classified
according to a system developed by Mushin and colleagues. 4
Technological advances made during the past 50 years have
dramatically changed the way ventilators operate, and these changes
required an updated approach to ventilator classification. The
following discussion is based on an updated classification system
proposed by Chatburn. 1 Chatburn’s approach to classifying
ventilators uses engineering and clinical principles to describe
ventilator function. 2 Although this classification system provides a
good foundation for discussing various aspects of mechanical
ventilation, many clinicians still rely on the earlier classification
system to describe basic ventilator operation. Both classification
systems are referenced when necessary in the following discussion to
describe the principles of operation of commonly used mechanical
ventilators.
98. Internal Function
A ventilator probably can be easily understood if it is pictured as a
“black box.” It is plugged into an electrical outlet or a high-pressure
gas source, and gas comes out the other side. The person who
operates the ventilator sets certain dials or a touch panel on a control
panel (user interface) to establish the pressure and pattern of gas flow
delivered by the machine. Inside the black box, a control system
interprets the operator’s settings and produces and regulates the
desired output. In the discussion that follows, specific characteristics
of the various components of a typical commercially available
mechanical ventilator are discussed. Box 2.1 provides a summary of
the major components of a ventilator.
99. Power Source or Input Power
The ventilator’s power source provides the energy that enables the
machine to perform the work of ventilating the patient. As discussed
in Chapter 1, ventilation can be achieved using either positive or
negative pressure. The power used by a mechanical ventilator to
generate this positive or negative pressure may be provided by an
electrical or pneumatic (compressed gas) source.
Electrically Powered Ventilators
Electrically powered ventilators rely entirely on electricity from a
standard electrical outlet (110–115 V, 60-Hz alternating current [AC]
in the United States; higher voltages [220 V, 50 Hz] in other countries),
or a rechargeable battery (direct current [DC]) may be used. Battery
power is typically used for a short period, such as for transporting a
ventilated patient, or in homecare therapy as a backup power source if
the home’s electricity fails.
An on/off switch controls the main electrical power source. The
electricity provides the energy to operate motors, electromagnets,
potentiometers, rheostats, and microprocessors, which in turn, control
the timing mechanisms for inspiration and expiration, gas flow, and
alarm systems. Electrical power also may be used to operate devices
such as fans, bellows, solenoids, and transducers. All these devices
help ensure a controlled pressure and gas flow to the patient.
Examples of electrically powered and controlled ventilators are listed
in Box 2.2.
Pneumatically Powered Ventilators
Current-generation intensive care unit (ICU) ventilators are typically
pneumatically powered devices. These machines use one or two 50-
psi gas sources and have built-in internal reducing valves so that the
operating pressure is lower than the source pressure.
100. Pneumatically powered ventilators are classified according to the
mechanism used to control gas flow. Two types of devices are
available: pneumatic ventilators and fluidic ventilators. Pneumatic
ventilators use needle valves, Venturi entrainers (injectors), flexible
diaphragms, and spring-loaded valves to control flow, volume
delivery, and inspiratory and expiratory function (Fig. 2.1). The Bird
Mark 7 ventilator, which was originally used for prolonged
mechanical ventilation, is often cited as an example of a pneumatic
ventilator. These devices also have been used to administer
intermittent positive pressure breathing (IPPB) treatments. IPPB
treatments involve the delivery of aerosolized medications to
spontaneously breathing patients with reduced ventilatory function
(e.g., chronic obstructive pulmonary disease [COPD] patients).
BOX 2.1 Components of a Ventilator
1. Power source or input power (electrical or gas source)
a. Electrically powered ventilators
b. Pneumatically powered ventilators
2. Positive or negative pressure generator
3. Control systems and circuits
a. Open-loop and closed-loop systems to control
ventilator function
b. Control panel (user interface)
c. Pneumatic circuit
4. Power transmission and conversion system
a. Volume displacement, pneumatic designs
b. Flow-control valves
5. Output (pressure, volume, and flow waveforms)
BOX 2.2 Examples of Electrically Powered
101. Ventilators
LTV 1000 and LTV 1150 Ventilators (Becton, Dickinson, and
Company, Franklin Lakes, N.J.)
Newport HT70 (Newport Medical Instruments, Costa Mesa,
Calif.)
Fluidic ventilators rely on special principles to control gas flow,
specifically the principles of wall attachment and beam deflection. Fig.
2.2 shows the basic components of a fluidic system. An example of a
ventilator that uses fluidic control circuits is the Bio-Med MVP-10.
(Fluidic circuits are analogous to electronic logic circuits.) Fluidic
systems are only occasionally used to provide ventilation to patients
in the acute care setting. 3
Most pneumatically powered ICU ventilators also have an electrical
power source incorporated into their design to energize a computer
that controls the ventilator functions. Notice that the gas sources,
mixtures of air and oxygen, supply the power for ventilator function
and allow for a variable fractional inspired oxygen concentration (FI O
2). The electrical power is required for operation of the computer
microprocessor, which controls capacitors, solenoids, and electrical
switches that regulate the phasing of inspiration and expiration, and
the monitoring of gas flow. The ventilator’s preprogrammed
ventilator modes are stored in the microprocessor’s read-only memory
(ROM), which can be updated rapidly by installing new software
programs. Random access memory (RAM), which is also incorporated
into the ventilator’s central processing unit, is used for temporary
storage of data, such as pressure and flow measurements and airway
resistance and compliance (Key Point 2.1).
102. FIG. 2.1 The Bird Mark 7 is an example of a pneumatically powered
ventilator.
Courtesy CareFusion, Viasys Corp., San Diego, Calif.
Key Point 2.1
Pneumatically powered, microprocessor-controlled ventilators rely
on pneumatic power (i.e., the 50-psi gas sources) to provide the
energy to deliver the breath. Electrical power from an alternating
current (AC) wall-socket or from a direct current (DC) battery power
source provides the energy for a computer microprocessor that
controls the internal function of the machine.
Case Study 2.1 provides an exercise in selecting a ventilator with a
specific power source.
103. Case Study 2.1 Ventilator Selection
A patient who requires continuous ventilatory support is being
transferred from the intensive care unit to a general care patient
room. The general care hospital rooms are equipped with piped-in
oxygen but not piped-in air. What type of ventilator would you select
for this patient?
Positive and Negative Pressure Ventilators
As discussed in Chapter 1, gas flow into the lungs can be
accomplished by using two different methods of changing the
transrespiratory pressure gradient (pressure at the airway opening
minus pressure at the body surface [Pawo − Pbs]). A ventilator can
change the transrespiratory pressure gradient by altering either the
pressure applied at the airway opening (Pawo) or the pressure around
the body surface (Pbs). With positive pressure ventilators, gas flows
into the lung because the ventilator establishes a pressure gradient by
generating a positive pressure at the airway opening (Fig. 2.3A). In
contrast, a negative pressure ventilator generates a negative pressure
at the body surface that is transmitted to the pleural space and then to
the alveoli (see Fig. 2.3B).
104. Control Systems and Circuits
The control system (control circuit), or the decision-making system
that regulates ventilator function internally, can use mechanical or
electrical devices, electronics, pneumatics, fluidics, or a combination of
these.
Open-Loop and Closed-Loop Systems to
Control Ventilator Function
Advances in microprocessor technology have allowed ventilator
manufacturers to develop a new generation of ventilators that use
feedback loop systems. Most ventilators that are not microprocessor
controlled are called open-loop systems. The operator sets a control
(e.g., tidal volume), and the ventilator delivers that volume to the
patient circuit. This is called an open-loop system because the
ventilator cannot be programmed to respond to changing conditions.
If gas leaks out of the patient circuit (and therefore does not reach the
patient), the ventilator cannot adjust its function to correct for the
leakage. It simply delivers a set volume and does not measure or
change it (Fig. 2.4A).
Closed-loop systems are often described as “intelligent” systems
because they compare the set control variable with the measured
control variable, which in turn allows the ventilator to respond to
changes in the patient’s condition. For example, some closed-loop
systems are programmed to compare the tidal volume setting with the
measured tidal volume exhaled by the patient. If the two differ, the
control system can alter the volume delivery (see Fig. 2.4B). 5-7
Mandatory minute ventilation is a good example of a closed-loop
system. The operator selects a minimum minute ventilation setting
that is lower than the patient’s spontaneous minute ventilation. The
ventilator monitors the patient’s spontaneous minute ventilation, and
if it falls below the operator’s set value, the ventilator increases its
output to meet the minimum set minute ventilation (Critical Care
106. FIG. 2.2 Basic components of fluid logic (fluidic) pneumatic
mechanisms. (A) Example of a flip-flop valve (beam deflection). When
a continuous pressure source (PS at inlet A) enters, wall attachment
occurs and the output is established (O2). A control signal (single gas
pulse) from C1 deflects the beam to outlet O1. (B) The wall attachment
phenomenon, or Coandă effect, is demonstrated. A turbulent jet flow
causes a localized drop in lateral pressure and draws in air (figure on
left). When a wall is adjacent, a low-pressure vortex bubble (separation
bubble) is created and bends the jet toward the wall (figure on right).
From Dupuis YG. Ventilators: Theory and Clinical Applications, 2nd ed.
107. St. Louis, MO: Mosby; 1992.
CRITICAL CARE CONCEPT 2.1 Open-Loop or
Closed-Loop
A ventilator is programmed to monitor SpO2. If the SpO2 drops below
90% for longer than 30 seconds, the ventilator is programmed to
activate an audible alarm that cannot be silenced and a flashing red
visual alarm. The ventilator also is programmed to increase the
oxygen percentage to 100% until the alarms have been answered and
deactivated. Is this a closed-loop or an open-loop system? What are
the potential advantages and disadvantages of using this type of
system?
Control Panel (User Interface)
The control panel, or user interface, is located on the surface of the
ventilator and is monitored and set by the ventilator operator. The
internal control system reads and uses the operator’s settings to
control the function of the drive mechanism. The control panel has
various knobs or touch pads for setting components, such as tidal
volume, rate, inspiratory time, alarms, and FI O 2 (Fig. 2.5). These
controls ultimately regulate four ventilatory variables: flow, volume,
pressure, and time. The value for each of these can vary within a wide
range, and the manufacturer provides a list of the potential ranges for
each variable. For example, tidal volume may range from 200 to 2000
mL on an adult ventilator. The operator also can set alarms to respond
to changes in a variety of monitored variables, particularly high and
low pressure and low volume. (Alarm settings are discussed in more
detail in Chapter 7.)
